Thermistor



Nov. 24, 1964 D. W. BARGEN THERMISTOR Filed May 7, 1963 INVEN TOR. David W. Burgen STEP l STEP 2 STEP 3 STEP 4 SSS-BSE STEP 5 ,allowing it to cool.

United States Patent O This invention relates to packaging of thermistors and other semiconductor devices, and in particular relates to an improved package for thermistors adapted to be embedded in the vwindings of dynamoelectric machines to provide over-temperature protection.

The rwindings of motors and other dynamoelectric machines should be protected from temperatures that will i unduly shorten their lite. The conditions that are most likely to cause a motor to overheat are: overloads which produce high running currents in the windings; stalling of the rotor; and loss of a phase in polyphase machines. @ver-temperature protection for motors, and other machines and devices having windings susceptible to overheating, may be provided by means of a thermistor placed in the winding and connected in a control circuit. The resistance of the thermistor undergoes aV large and abrupt change when the winding reaches a limiting temperaturesaely below the inherent temperature limitation of the motor. This resistance change causes the Vcontrol circuit to interrupt the circuit path kthrough which power is ,supplied to the motor sol that the motor will stop,

lt may be restarted automatically or manually.

rl`he resistance, of the thermistor may either increase Ior ecrease with increasing temperature, ie., it may have l contacts, and lead wires connected to the contacts. Clears ly, anything added to protect the thermistor, such as plastic encapsulation or a can, slows down its thermal response because heat fromthe windings must be transferred to they semiconductorV through the encapsulation. ln some casesy the problem may be alleviated yby reducing theV size ofthe thermistor. However, most' motor protection applications require that the thermistor dissipate substantial amounts of power, and in vk'view of thisllimitation,

' the size of the thermistor cannot be reduced to the extent required to provide the desired degree lof thermal response. Ontlie other hand, a bare therrnistt'rrfv (i.e., unencapsulated) is not necessarily the best approach for motor protection applications.' placed in the winding, the

bare thermistor may have poor thermaly contact with individualwires suchthat optimum heat transfer conditions arenot achieved. rlhe responseiis wimproved if the bareV *thermistor is soldered directly toy they copper ofthe windings c i l- ,becauselthe solder'connectiori has low thermalresistance. j] n, f .The solder conneetion'taps thethermistor into the winding lll-Pafk and lower ffl-Ces.:

'such lthat electrical power-tor the control-circuit is derivedk n, fromthe windin'gand there,v is Vno separate power supply. I 'for the cohtrol'circuit. .p y

` y"Unfortunately,"a'number of practical problems have lbeen encounteredwith this soldermmounting approach,

l ;;Fi1st,there is a ,possibility for error' sincefthev thermistor `must bet'apped intothe right voltage point' inthe winding. n f

As a practicalv matter, thisL means that the etiectiveness of the motor g protection `depends vcritically on the .relis kabilit'yfottheindividualiV assembly workerfwho accom-`A arsasss Patented Nov. 24, i964 ice plishes the soldering. ln additiomit takes some finesse Y to do the soldering without cracking the semiconductor element, and this is not accomplished easily on the production line forassembling the motor. Improper soldering can also change the thermistor characteristics through chemical contamination and other causes, thereby adversely ailecting the system operation. As previously mentioned, it is desirable in some applications for the thermistor to have a large change of resistance for a small `change of temperature at the limiting temperature range so that a definite switching action will be achieved. Consequently, the range and slope of the thermistors characteristic curve should be maximized. In general, it has been found that soldering a bare therm'istor to the winding is not the best solutionfor increasing the thermal response of the ther-mister.

lt is an object' of this invention to shorten the response time of thermistors without` unduly limiting their power dissipation capabilities.

Another object of the invention is to provide a thermistor having a faster thermal response than unsoldered bare thermistors and plastic encapsulated thermistors of equivalent size.

It is also an object of the invention to provide a metal package for thermistors which is especially adapted to be embedded in an electrical winding, and which does not have to be soldered to the winding inorder to provide adequate thermal response.- n ln its essence, the invention is a thermistor with ya sealedy met'al -foil package having an eirten'dedfmetal surface for increasing the area of thermal contact between the thermistorand a windingin which it is to be embedded; The extended metal surface is deformable such that it will conform to the wires of the winding in which it is placed, and the metal of the package is thin, all of which y contributes to a fast thermal response which is charac" teristic of the metal-foil packaged thermistor. A particular embodiment of the invention has apiece'of foil ysoldered to each side of the semiconductor element with the foils aligned with each other and with their borders exy tending beyond the edge of the semiconductor element. The extension or border-'portions of thefoils are sealed by va separator impregnated with thermosetting kresin material which is placed between them, and the effectiveness of the seal is kenhanced by the relatively large area of Contact which exists between the resin and the foils.

Lead wires are solderedto ,the foils such that the foils serve both as 'electrical and thermal connections to the semiconductor element. The separator projects slightlyy beyond the boundary of the foils, so the package has nok sharp metal corners which might damage the insulation of the winding in which, the thermistor is lultimately embedded, a factor which improves the reliability of a motor or other machine in which the thermistor is used toprovide thermal protection. y

Referring to -the drawings, a metal-foil packaged thermistorwhich constitutes a preferred embodiment of .theginvention isrshown in perspective in FIG. l and in an `exploded viewiin yFlGnZ.. The. thermistor Il@ includes a f semiconductor element *11. (FIG. 2) in the form of a cylindricalffpellet which'hasfmetal contacts' l2 and` 13 on its The kcylindrical shape of the pely let is convenient but not es`sential.*- Positive temperature coefficient (PTC) therristorsr` are preferred for some c thermal-protection applications, and thetmetal-foilpackage of FIGS. l and 2 has beenfapplied snccessfuilyto thermis-- tors of 4this type. The utility of the package is not'limited stofPlfC ,thermistora however. ,tCerami'cV semiconductive compositionsand suitable metalcontacts forFTC therrriis-,r` y f tors have been described in the literaturefor example inan article entitled @Oxide 'lhermistors WithfLarge. Posttive Ternperaturea.Coeicientsv byH. AgrSauerfand l Flaschen, erarnic lndustry, vol. 66, page 95 (1956) and another article entitled Choice of Electrodes in Study and Use of Ceramic Semiconducting Oxides by H. A. Sauer and S. S. Flaschen, The American Ceramic Society Bulletin, vol. 39, No. 6, pages 304-306, lune i960.

Examples of suitable compositions for the ceramic semiconductor element of FTC thermistors are as follows:

BamSfmLa.ooaTrusOs 321.8003?.isvLanoSTLosOs Ba.95oSf.c47La.oosTi.0603 BaroTLosSbnosOs Satisfactory metal contacts to the semiconductor element have been made by electroless plating of nickel on opposite faces of the element.

The package for the thermistor illustrated in FlGS. l and 2 consists oli' two foils )i4 and l5 preferably oi copper which are soldered at their central parts to the metal contacts l2 and i3, and which are sealed around their peripheral portions by means of a separator lo of fibrous material, eg., glass liber, impregnated with thermosetting resin. The term toil as used in this specification and the claims which follow means a sheet of metal having a thickness of about l() mils or less. The foils i4 and l5 are usually circular disks, but the circular shape is not essential. Lead wires 17 and i8 are soldered respectively to the :toil disks t4 and l5 to provide electrical connections to the element ll.

The separator la projects slightly beyond the rims of the foil disks i4 and l5 as shown in FIGS. l and 2 such that the package has no sharp metallic edge at its perimeter. When the thermistor is embedded in a winding of a motor, for example, the wires lll and 2? which are in contact with the thermistor as shown in FlG. 3 do not cross a sharp metallic edge capable of cutting their insulation, so there is no tendency for the thermistor to damage the insulation on the wires.

The toil disks i4 and l5 have been formed around the edge of the ceramic pellet so that the pellet is more or less wrapped in the foil. The separator lo gives the peripheraleortion of the package suficient stiffness that it Will hold its shape, but the package can be deformed easily so that it willconforrn to the wires of the winding in which it is ultimately embedded, as previously noted. Since the package has a considerably larger overall area than the pellet itself, and since it conforms to the surrounding wires, it is easy to provide good thermal contact between the thermistor and a winding. The package itself has low thermal resistance due to the thinness or" the foil, typically about two mils, and the thermal time constant of the tiermistor when properly installed in a motor winding is approximately three seconds. By way of comparison, competitive commercially available thermistors having a thin plastic coating directly on the pellet have a thermal time constant, when embedded in a winding, of approximately l2 seconds.

FIG. 4 shows the steps of a preferred method of fabricating the thermistor lll of FlGS. l and 2. Two foil disks 22 and 23 are cut from .G02 inch copper sheet to a diameter of about seven-eighths inch. rfhe disks should be kept as liat as possible and free from fingerprints or other contamination. rlhe disks 22 and 23 are assembled with solder preforms 25 and a semiconductive pellet lll in a soldering jig (not shown) such that the foil disks are on opposite sides of the pellet with solder preforrns between the pellet and the copper disks. An exploded View of this assembly is shown at Step l of FIG. 4. lf desired, the pellet may be pre-tinned to provide solder on its Contact faces l2 and i3 rather than using separate solder preforms. A drop of suitable liux, such as 20% hydrochloric acid, is placed on the solder or `on the center of the foil disks temperature above the melting point of the solder, for example, about 275 C., and is then cooled to solidify the solder joints. A small lillet of solder should be visible on both sides of the pellet, and the pellet and disks 22 Zand 23. The assembly is'heat'ed at .a

l and 23 should be concentric after the soldering step as shown at Step 2 of FIG. 4.

A glass-liber sheet 24 impregnated with a thcrmosetting epoxy resin is then inserted between the extended portions of the disks 22 and 23 and encircles the pellet ll as shown at Step 3 in FlG. 4. The sheet has a hole at its center to accommodate the pellet, and it is slit radially so that it may be slipped around the pellet. lf desired, two such sheets may be placed with the slits at a angle with respect to each other in order to insure a tight seal. The r sulting assembly is then placed in a hydroforming press between two rubber blocks which have faces parallel to each other. The press is closed, and the rubber blocks form the extended portions of the foil disks around the sides of the pellet ll and against the epoxy-impregnated sheet 24 producing an assembly with a conliguration as shown at Step 4 in FlG. 4. Note that the peripheral portions of the disks and the insulating sheet 24 form a nlike projection which increases the thermal contact area of the thermistor over that of the pellet ll.. This assernbly is placed in a laminating press heated at a temperature of approximately 325 C. to 350 C. in order to seal the package. ln a specific example, the assembly remains in the laminating press for about live minutes in order to advance the cure of the epoxy resin until the copper material adheres rrnly to the insulating sheet. The epoxy resin of the insulating sheet is initially partially cured, and the lamination step advances the cure sufficiently to produce good adherence between the copper and the insulating sheet.

Excess flash is trimmed from the package such that the nal overall diameter is about one inch. The final configuration of the package is shown at Step 5 in FIG. 4. The insulating sheet projects from the rim of the copper disks a radial distance of about one-sixteenth inch. The leads 26 and 27 are soldered to the disks 22 and 23 respectively by conventional solder techniques. As a final processing step, the package may be coated with a thermosetting varnish if desired, but the varnish coating is not essential for all applications. The leads are usually twisted together to provide added mechanical strength and ease of handling.

The following speciications for a speciic embodiment of the invention are given by way of illustration, and are not intended to limit the invention in any way.

Transition Temperature C. Resistance at 25 C 50 ohms Temperature Coetiicient of Resistance 13%/ C. Thermal Time Constant-- In still air at 25 C 3() sec. Embedded in winding 3 sec.

Ther invention provides a thermistor device, and a metal-foil package for it, which has a thermal response time faster than potted or bare tlzermistors, and yet which is capable of dissipating a reasonable amount of power. The foil packaged thermistor is simple, rugged and reliable, and can easily be embedded in a motor winding or other winding without soldering the thermistor to the wires of the winding. The thermistor is particularly adapted for over-temperature protection of motors, but is suitable for other applications as well.

I claim:

, l. A thermistor device comprising a solid thermistor body of semiconductive material having a specied temperature coefcient of resistance, first and second foil means connected vto opposite sides of said body, said foil from said body substantially parallel to each otherV and extending around the perimeter of said body 'to increase the thermal contact area of said thermistor over that of V'said body, insulating means separating said border portions of said foil means fand adherently lbonded to-said border portions for sealing said enclosure, and electrical connector means connected to said foil means.

2. A thermistor device comprising a solid thermistor body of semiconductive material having a specified temperature coefficient of resistance, rst and second foils connected to opposite sides of said body, at least one of said foils being formed partially about said body toward the other lof said foils such that said foils nearly enclose said body, said foils having border portions projecting from said body substantially parallel to each other and extending around the perimeter of said body to increase the thermal contact area of said thermistor over that of said body, flexible insulating means separating said border portions of said foils from each other and adherently bonded to said border portions for sealing the enclosure formed by said foils, and electrical connector means connected to said foils and having portions available for making external electrical connections to said thermistor.

3. A thermistor comprising a solid body of semiconductive material having a specied tempenature coeiiicient of resistance, first and second metal-foil disks soldered to opposite sides of said semiconductive body providing thermally conductive and electrically conductive connections to said body, said metal-foil ydisks each having a portion partially wrapped about said body, and said disks having peripheral portions projecting radially from and circumscribing said body so as to increase the thermal contact area of said thermistor over that of said semiconductive body, an insulating separator between and adhering to said peripheral portions of said disks and encircling said semiconductive body, said separator having enough flexibility to allow said peripheral foil portions to conform to wires of a winding when said thermistor is inserted in such winding, and electrical lead means connected to said metal-foil disks.

4. A thermistor comprising a pellet of semiconductive material having a specified temperature coeiiicient of resistance, first-and second foils soldered to opposite sides of said pellet providing thermally conductive and electrically conductive connections to said pellet, said foils having portions formed partially about said pellet and extending outward from said pellet forming a iin-like projection extending entirely around said pellet, an insulating separator between and adhering to the outwardly extending 6 portions of said foils providing a sealed package for said pellet, and electrical connector means connected to said foils. A

,5. A thermistor comprising a body of semiconductive material having a specified temperature coeilicient of resistance, first and second foils connected toopposite sides of said body, at least one of said foils being partially wrapped about said body toward the other of said foils such that said foils nearly enclose said body, and said foils having border portions projecting from said body substantially parallel to each other so as to increase the thermal cont-act area of said thermistor over that of said body, an insulting separator between and adherent to said border portions of said foils for sealing the enclosure formed by said foils, said separator extending slightly beyond the outer boundary of said foils and being flexible such that said thermistor will conform to the wires of an electrical winding when embedded therein, and electrical connector means connected to said foils.

6. A thermistor comprising a body of semiconductive material having a specified temperature coetcient of resistance, first and second foils connected to opposite sides of said body, at least one of said foils being formed partly around said body toward the other of said foils such that said foils nearly enclose said body, and said foils having border portions extending from said body substantially parallel to each other so as to increase the thermal contact area of said thermistor over that of said body, an insulating separator comprised of a fibrous sheet impregnated with thermosetting resin material, with said separator lying between and adhering to said border portions of said foils yfor sealing the enclosure formed by said foils and insulating said border portions from each other, said separator and said foils being suciently ilexible to allow said thermistor to conform to the wires of an electrical winding when embedded therein, and electrical connector means connected to said foils.

References Cited in the lile of this patent UNITED STATES PATENTS 2,707,223 Hollmann Apr. 26, 1955 2,880,295 Huffman Mar. 31, 1959 3,085,216 Melton Apr. 9, 1963 

1. A THERMISTOR DEVICE COMPRISING A SOLID THERMISTOR BODY OF SEMICONDUCTIVE MATERIAL HAVING A SPECIFIED TEMPERATURE COEFFICIENT OF RESISTANCE, FIRST AND SECOND FOIL MEANS CONNECTED TO OPPOSITE SIDES OF SAID BODY, SAID FOIL MEANS BEING FORMED PARTIALLY ABOUT SAID BODY TOWARD EACH OTHER AND THEREBY FORMING AN ENCLOSURE FOR SAID BODY, AND SAID FOIL MEANS HAVING BORDER PORTIONS PROJECTING FROM SAID BODY SUBSTANTIALLY PARALLEL TO EACH OTHER AND EXTENDING AROUND THE PERIMETER OF SAID BODY TO INCREASE THE THERMAL CONTACT AREA OF SAID THERMISTOR OVER THAT OF SAID BODY, INSULATING MEANS SEPARATING SAID BORDER PORTIONS OF SAID FOIL MEANS AND ADHERENTLY BONDED TO SAID BORDER PORTIONS FOR SEALING SAID ENCLOSURE, AND ELECTRICAL CONNECTOR MEANS CONNECTED TO SAID FOIL MEANS. 